The present invention relates to integrated chip (IC) air core gage controls and, more particularly, to a serial interface circuit apparatus for driving an air core gage from a serial data input derived from any source.
Air core gage controllers and control methods have a variety of applications, often in less than ideal environments. Some applications include automotive, aircraft and marine instrument panels, as well as industrial and medical instruments. Typically, an air core gage is used along with a pointer and dial assembly to display vehicle speed. The air core gage is driven by a signal from an electric speedometer circuit board.
In the basic operation of an air core gage, a sinusoidal signal is transmitted to the circuit assembly by a permanent magnet speed sensor. The sinusoidal signal is processed to provide the proper signals necessary to drive the air core gage mechanism. A conventional air core gage has a horizontal coil and a vertical coil. Current flowing through each of these coils causes a magnetic flux field to be induced. As the current through the coils varies, the induced flux field varies.
In a speedometer air core gage assembly application, the varying flux field causes a magnet and vehicle speed indicator assembly to rotate inside a cavity. By controlling the magnitude and direction of the current through each coil, a flux field can be established to control the resultant angle of rotation of the magnet and spindle assembly. With proper control of current magnitude and direction, accurate vehicle speed is indicated by the air core gage and dial assembly. To achieve proper control of the current flow, the speedometer circuitry utilizes the transmitted speed signal to determine the octant the air core gage should be operating within.
One type of speedometer system that utilizes an air core gage is a quartz electric speedometer. The quartz electric speedometer includes conventional dials, pointers and odometer wheels to indicate vehicle speed and mileage. Improved drive methods are used to drive these components in place of the standard drive system presently used with mechanical methods being operated with mechanical drive gear speedometer systems. To provide accurate vehicle information, the quartz electric speedometer utilizes a clock signal supplied by a quartz crystal along with integrated electronic circuitry to process a speed signal. The vehicle speed signal is provided by a transmission mounted permanent magnet speed sensor and is used to drive the air core gage mechanism and odometer stepper motor. This system eliminates the need for a flexible shaft speedometer cable used with the mechanical speedometer and provides more accurate and precise vehicle speed information to the driver.
Many vehicles manufactured today are provided with a two chip speedometer air core gage instrument system which includes a logic chip and a driver chip. The circuitry of one such logic chip is disclosed in U.S. Pat. No. 4,051,434 to Sweet, issued Sept. 27, 1977, and assigned to the assignee of the present invention. The circuitry disclosed in the Sweet patent consists of a bit serial computation engine which performs a Vehicle Speed Signal (VSS) frequency to miles per hour (MPH) calculation and generates the control signals needed by the driver chip.
Another alternative means for providing air core gage control of display systems involves microprocessor supervision. Microprocessor supervised systems are host dependent, meaning a microprocessor or similar host is needed to apply an input frequency to drive the air core gage controller. Current methods of microprocessor supervised air core gage control require the generation of pulse-width-modulation (PWM) signals instead of using an analog frequency conversion method. Due to limited chip resources, current methods of microprocessor air core gage control require a custom chip to generate signals. However, microprocessor supervision yields improved accuracy given the additional computational resources available. Additionally, host dependent systems are sometimes preferred when multiple air core gage applications exist, as in a system wherein the speedometer, tachometer and odometer each have a separate air core gage assembly which must be driven. When multiple hosts share data in a vehicle, a host dependent mode of operation may be preferred to save on wiring costs and to minimize connection problems that arise when a system is extensively wired.
Digital control requires digital-to-analog conversion of a calculated result into a drive voltage to be applied to the air core gage coils. Current methods for this conversion have been implemented with a 512 Hertz (Hz) pulse-width-modulated drive signal. However, at this low frequency, audible noise may be generated in the air core gage coils, thus mandating the use of a higher frequency, greater than 20 KHz, which would be above the audible range. Also, an air core driver chip is needed for each gage in the display, adding to the overall cost of such a system.
Hence, it would be desirable to provide a method and apparatus for air core gage control which would eliminate the need for additional custom chips and additional driver chips. It would also be desirable to provide a serial engine circuit which would allow two modes of operation to be accomplished, single chip and host dependent, even where the host dependent mode has multiple chips, each driving an air core gage assembly. Finally, it would be desirable to provide a method of air core gage drive control having host compatibility, thereby opening new avenues for advanced air core displays and integrated manufacturing environments for analog instrument panels.